The purpose of overload protection is to prevent the temperature of the motor and/or the drive from exceeding levels that would cause damage. Overload protection trips the drive or relieves the load or alarms the operator or takes other appropriate action when those critical temperatures are approached.
In the prior art such protection is provided by various methods, including the following:
1. An overtemperature switch is mounted in the motor or on the drive at a point where the critical temperature would occur, and is set to actuate when the critical temperature is approached; actuation initiates appropriate protective action.
2. A thermal overload relay is actuated by thermal elements that are conducting motor current or drive load current. The elements heat in proportion to the square of the load current, and melt (after some time delay upon occurrence of an overload), tripping the relay. This is referred to as an I.sup.2 T relay because the temperature of the protected device is simulated by the I.sup.2 T heating of the thermal elements. I.sup.2 T is a quantity related to the heat power dissipation of the motor or inverter, in which I represents load current and T represents time.
3. An electronic overload sensor is provided comprising a multiplier for squaring the current, an integrating filter for delay, and a comparator for comparing the resulting indication of I.sup.2 T level with a predetermined threshold and activating the protection when the threshold is reached.
4. A microprocessor having an I.sup.2 T algorithm computes the I.sup.2 T of the protected device based on sensed load current, compares it with a predetermined threshold value and initiates protective action when the predetermined value is reached.
Generally speaking, the first two approaches increase the manufacturing cost of a variable speed drive system because they necessitate additional equipment; the third and fourth do not. The protection provided by many such prior art systems is not very precise because they rely only upon I.sup.2 T simulation, which alone does not give a true representation of temperature.